The optical channel (OCh) overhead is a communication channel optically "in band" with an associated OCh, which can be generated at the channel transmitter and read at any point in the optical network where the OCh is optically separated from other optical channels (at muxes, demuxes, optical network elements with tunable bandpass filters and optical channel receivers). The OCh overhead is required to be in band optically in order to be able to trace the OCh through wavelength selective devices. Depending on the network application, the OCh overhead can be used in channel identification for optical trace connectivity verification; it can also be employed as a communication channel for optical protection or for remote monitoring between transmitters and receivers. Moreover, the OCh overhead permits software downloads from optical channel transmitters to listening devices.
The optical channel overhead is typically a digital message signal s(t) that is carrier modulated about a carrier one, yielding a carrier modulated OCh overhead message signal q(t), and multiplied by a factor (known as the modulation depth, m), and which optically modulates the OCh payload in the optical channel. FIGS. 1A and 1B show how the oCh payload p(t) and overhead m.multidot.q(t) (m multiplied by q(t)) make up an optically modulated signal v(t), in the time and frequency domains, respectively. The OCh payload p(t) is typically formed by a series of high-speed pulses of light of intensity Pav or 0 depending on the "non-return-to-zero" (NRZ) value (1 or -1 respectively) of the transmitted digital data d(t). The digital data d(t) generally has a bit rate that may exceed 10 gigabits per second (Gbps), and tlin OCh payload is typically formatted as an OC-192 signal.
An optical attenuator 302 will then modulate p(t) with the OCh overhead m.multidot.q(t) so as to give an optically modulated signal v(t) according to the following equation: EQU v(t)=[1+m.multidot.q(t)]p(t)=Pav+[m.multidot.q(t)+m.multidot.q(t)d(t)+d(t)] Pav.
It can be shown that v(t) ranges from 0 to 2.multidot.Pav.+-.m.vertline.q(t).vertline.Pav, i.e., the average value of the peaks of v(t) have an amplitude of 2.multidot.Pav and the peaks of v(t) have an envelope of amplitude m.multidot.q(t)Pav. The oCh overhead m.multidot.q(t) comprises a lower frequency analog (or digital) carrier modulated OCh overhead message signal q(t) which is scaled by a modulation depth, or index, m. The signal q(t) has a bandwidth (or bit rate) on the order of several dozen kHIz (or kbps), and is a carrier modulated version of a digital OCh overhead message signal s(t), not shown in FIG. 1A. The value of m will determine the amplitude of the variation in the peaks of v(t) relative to the average value of the peaks of v(t). In prior art systems, the value of m is typically set to anywhere from 0.5% to 5%, and is fixed for a given fiber optic system.
In the frequency domain, the OCh payload p(t) is seen as comprising a relatively flat data portion 101, and a periodic set of peaks 102, occurring at 8 kHz intervals. The peaks 102 are due to the 8 kHz periodic nature of data frames used in typical digital optical networks, such as SONET (synchronous optical network) and SDH (synchronous digital hierarchy). The OCh overhead m.multidot.q(t) is seen as a localized hump 103 centered about a carrier tone 106 at 202 kHz and rising above the flat portion 101 of the OCh payload data spectrum 101. the bit rate of the och overhead message signal prior to carrier modulation is represented by the width 104 of the hump 103. The modulation depth can also be viewed as the peak power of the carrier tone 106 relative to that of the repetitive peaks 102.
The main requirement of OCh overhead implementation is to support the aforementioned range of functions by providing an adequately high OCh overhead message signal bit rate, while keeping an acceptable bit-error rate (BER) of the OCh overhead message signal. In addition, one must attempt to limit the degradation incurred on the OCh payload by appropriately choosing the modulation depth.
Typically, peaks in the power spectral density of the Och payload (such as the peaks 102 seen in FIG. 1B) represent the dominant threat to maintaining a low BER of the OCh overhead message signal s(t). The spectral peaks, which may vary from system to system (and in some cases may be unknown or slowly varying), interfere with the carrier tone 106 and message signal spectrum 103 of the OCh overhead and must be avoided. Prior art approaches fail to provide robust protection against periodic interference in the frequency domain. For example, the usage of spread spectrum techniques combined with subcarrier modulation attempts to exploit the capability of spread spectrum to mitigate the effects of interference. However, the presence of multiple harmonics from an interfering framed payload severely reduces this capability. Other prior art methods generally rely on a fixed frequency for the OCh overhead carrier tone 106, which leads to deleterious consequences when the OCh payload has an arbitrary frame rate.
Another factor that affects performance (BER) of the OCh overhead is the modulation depth. Since the BER of the OCh overhead message signal s(t) decreases with an increase in the relative amount of optical. Power occupied by the OCh overhead, it is desirable to keep the modulation depth, M, large. On the other hand, too high a modulation depth causes degradation of the OCh payload. Clearly, there are conflicting requirements of keeping both the modulation depth and BER as low as possible. Quite often, the minimum modulation depth that can be achieved whilst keeping the BER to within a fixed upper bound depends on the distance between network elements or on properties of the network elements themselves and may even change as a function of time. Hence, it would be advantageous to use a variable modulation depth m(t) for the OCh overhead message so that the BER of the OCh overhead message signal and degradation of the OCH payload are kept to a minimum.
An ability to vary the bit rate of the OCh overhead message signal would provide a great deal of flexibility for meeting evolving OCh overhead functional requirements, as well as increased overhead capacity for applications with no supervisory optical channel.